For example, JP-A-2004-221370 discloses a semiconductor device having an IGBT element. Specifically, in JP-A-2004-221370, a P-type body region is formed on a surface part of an N-type drift region, and multiple trenches that reach the drift region by penetrating the body region. A gate insulation layer is formed on a wall surface of the trench, and a gate electrode is formed on the gate insulation layer.
The P-type body region is divided into multiple semiconductor regions by the trenches. In one semiconductor region, the P-type body region, a P+-type emitter region, and an N+-type emitter region are formed. In the other semiconductor region, the P-type body region, the P+-type emitter region, and an N-type hole stopper layer are formed. The hole stopper layer is separated from and is not contact with two trenches that form the other semiconductor region. A portion of the body region is located between the hole stopper layer and the trench.
Further, an interlayer insulator is formed to cover the trench, and an emitter electrode is formed to cover the interlayer insulator. Thus, the emitter electrode is in contact with the emitter region of each semiconductor region.
In such a structure, in the other semiconductor region, during ON operation of the IGBT element, electrons accumulate between the gate insulation layer and the hole stopper layer so that an inversion layer can be formed. Thus, the flow of holes accumulating in the drift region to the emitter electrode is reduced by the hole stopper layer. During OFF operation of the IGBT element, the electrons accumulating between the gate insulation layer and the hole stopper layer disappear so that the hole in the drift region can flow to the emitter electrode.
In the above conventional technique, a floating effect in the other semiconductor region is enhanced by increasing an impurity concentration in the hole stopper layer so that a carrier accumulation effect of the IGBT element can be increased. However, when the impurity concentration in the hole stopper layer is increased in order merely to increase the carrier accumulation effect, a trade-off between surge and loss of an IGBT and a breakdown resistance (e.g., Reverse bias safe operation area (RBSOA)) become concern.